Transmitting system



Dec. 12, 1933. D, G LITTLE 1,939,070

TRANSMITTING SYSTEM Original Filed May 15, 1926 2 Sheets-Sheet 1 5/05 E: c f/f/r."

llllll INVENTOR ponolal/ff/e.

ATTORNEY Dec. 12, 1933. D rrr 7 1,939,070

TRANSMITTING SYSTEM Original Filed May 15, 1926 2 Sheets-Sheet; 2

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I03 m fllm |m |n INVENTOR Dona/d G tiff/e.

'ATTORNEY Patented Dec. 12, 1933 TRANSMITTING SYSTEM Donald G. Little, Longmeadow, Mass., as'signor to WestinghouseElectric & Manufacturing Company, a corporation of Pennsylvania Original application May 15, 1926, Serial No. 109,277. Patent No. 1,712,572, dated May 14, 1929. Divided and this application January 10, 1929. Serial No. 331,586

9 Claims. 701.250-36) My invention relates to transmittingsystems, and it has particular reference to radio transmitting systems.

This application is a division of application,

Serial No. 109,277, filed May 15, 1926, patented May 14, 1929, No. 1,712,572. y

One object of my invention is to provide a radio transmitting system that will radiate a carrier wave having a constant frequency.

Another object of my invention'is to provide, in a radio transmitting system having a piezoelectric-crystal control for the oscillation generator, a high-power amplifier that will not oscillate at undesired frequencies,

Another object of my invention is to provide, in the power amplifier portion of a radio transmitting system of the type described, meansfor preventing feed-back from one unit of the amplifier to a preceding unit.

Another object of my inventionis to provide, for the master oscillator tube of a transmitting system of the type describedabove, means for controlling the amplitude of the oscillations gen erated. 7

Another object of my invention is to provide, in a transmitting system of the type described,

means for preventing individual amplifier tubes,

from oscillating at undesired frequencies. I

A still further objectof my invention is to provide, in a transmitting system of the type described, means for compensating against energy feed-back from the output circuit of an'amplifiertube to the input circuit thereof by reason of the inter-element capacities of the tube and the capacity existing between such tube elements and the ground. Other and more specific objects will be apparent from a consideration of the accompanying however, being prone to oscillate .at a variety of higher harmonics of the fundamental fre-- quency. of the crystal- In my co-pending application, Serial No. 78,115, filed Dec. 29, 1925, I have disclosed and claimed a system for taking advantage of the abovespecified peculiarity of crystal-controlled oscillators, in order to transmit on ultra-short wave lengths. I

'It is necessary, when employing a plurality of stages of amplification between the crystal-controlled tube and the antenna, orradiating structure, to entirely eliminate the feed-back of energy from one stage to a preceding stage. To a certain extent, this feed-back is prevented when a system according to the disclosure in my copending application is employed, since each stage is tuned to a frequency differing from that of the preceding stage. Nevertheless, when highpower. tubes are used, the various elements of which have a large capacity-to-ground, it has been found extremely difiicult to so neutralize an amplifier that it will be entirely free from any tendency toward self-oscillation.

My present invention provides means for posi-- tively and definitely controlling the generationof oscillations in amplifier circuits.v To do this, I- use, instead of the usual type of neutralizing. condenser, .a capacitive device which not only, exactly duplicates all the inter-electrode capaci-v ties of the tubes employed, but also duplicates the capacity-to-ground of such tubes. Preferably, the device should be evacuated, and provided with three electrodes, and I have found that, by using, as a neutralizing device, a tube of the same type as the tube being neutralized, I can attain substantially perfect results.

For a complete understanding of my invention, reference is made to the accompanying drawings, and to the following description.

- Figurel is a diagrammatic View of the circuits involved in a radio transmitting system arranged according to. my invention, 7

Figure 21s a diagram illustrating the various tube capacities which Ihave found itnecessary to neutralize, 1 2'1 Fig. 3 is a diagram illustrating. an, alternative method for neutralizing the low-powered,

crystal-controlled stage of a radio transmitting system. 1

Referring specifically to Fig. 1, a master-oscillator tube 1 having a grid 2, a filament 3 and a pass. Plate potential for the tube is obtained from a battery 10, a portion 12 of the battery 10 also providing grid-bias potential for the masteroscillator tube 1 and the next tube in the series.

A second amplifier tube 13, of considerably higher power than the oscillator tube 1, is provided, this tube being also provided with the usual filament 14, plate 15 and grid 16. The grid 16 is coupled to the output circuit 7, act the oscillator tube 1 through a condenser 17, and is supplied with bias potential from the battery 10 through a'radio-frequency choke coil 18.

Plate current for the amplifier tube 13 is supplied from a high-voltage source (not shown) through a radio-frequency'choke coil 19.

An oscillating circuit, comprising a plurality of condensers 20 and an inductor 22, is coupled to the plate 15 through a condenser 23, and an additional condenser 24 is connected between an in,- termediate point 25 on the inductor 22 and the filament 14. A connection is also made from the inductor 22, through a neutralizing condenser 26, to the gird 16 of the tube 13, the point 27 of connection to the inductor 22 being so chosen that it will, at all times, be at a potential with respect to the central filament tap 25 which is equal and opposite to that of the point at which the coupling condenser is connected. The pur pose of this will be later explained in connection with Fig. 3. g

A plurality oi. high-power amplifier tubes 2 are provided each having a grid 30, a filament 32- and a plate 33. The plates and grids of these power-amplifier tubes are arranged in parallel, the grids being conductively connected to the inductor 22 in the output circuit of the preceding tube 13. The grids 30 of these tubes are also connected to the filaments through a radio-irequen'cy choke 34, a resistor 35 and a keying arrangement 36, which will be described in detail later.

An oscillating circuit, comprising a plurality of condensers 37 and an inductor 38, is conductively connected tothe plates 33, direct current being fed to the plates through a radio-frequency choke 39. A condenser 4001 large capacity is connected between an intermediate point 41 on the inductor 38 and the filament circuit, to serve as a radio frequency by-pass. A plurality of neutralizing devices 42 are connected between a point 43 on the inductor 38 and the grids 30 of the power amplifier'tubes 29, in substantially the same man ner as is connected the condenser 26 in the preceding stage. an additional electrode 44, however, which electrode is connected to the filament circuit for reasons which will be later explained.

The last amplifier preferably comprises a plurality of extremely high-power water-cooled tubes 45, each having a filament 46, plate 47 and grid 48. The plates 47 of these tubes are arranged in parallel, as are also the grids, the latter being connected to the output inductor38 of the previous stage througha condenser 49, and'to the filament circuit through a choke coil 50,. a

taken to keep the losses in this circuit as low as possible.

The plates 4'7 of the amplifier tubes are conductively connected to theinductor 53, and the inductor is also coupled to an antenna through a plurality of coupling condensers 56.

An. intermediate point. 57 on theinductor 15-90 1 These devices 42 are provided with nected to the filament circuit through a condenser 58. Direct current for the plates of the amplifier tubes 45 passes through an iron-cored surge-choke 59 which prevents high transient potentials from being fed back to the source of plate potential.

A plurality of three-electrode neutralizing de vices 60 are connected between a properly chosen point 62 on the inductor 53 and the grids 48, in the same manner that the neutralizing devices 42 are arranged inv the preceding amplifier stage.

Filament potential for all the tubes thus far described is supplied from a single source (not shown) which may be either a transformer, a motor generator or a storage battery.

Signaling is accomplished by applying to the grids of the amplifier tubes a potential sufficient to cause them to block, or become non-conductive. When it is desired that they become conductive, the contact members 63 and 64 of a relay, or keying device 36, are closed, connecting the grids of the amplifier tubes directly to the filament circuit through the resistors 35 and 52. If it is desired to again block the tubes, the contact members 64 and 65 are closed, which connects the grids to the filament by way of a circuit which may be traced through the resistors 35, 52, the added resistor 66, a secondary 67 of a power transformer 68, and the path between a plate 69 and a filament 70 of a rectifier tube '72. The filament 70 of the rectifier is supplied with potential from ,the same source used toenergize the oscillator and amplifier tubes, and plate potential for the tube is supplied by the power transformer 68, the primary 73 of which may be connected across a 220 volt, 60 cycle supply. The'rectifier tube '70 builds up, on con-' denser 74, a charge with a voltage equal to the maximum voltage obtained across the secondary 67. This voltage is applied to the grids of tubes 29 and 45 when relay contacts 64 and 65 are closed, and is sufficient to cause them to com pletely block.

This blocking cannot be accomplished, how-- ever, when high-power, water cooled tubes are used, unless feed-back between stages is absolutely neutralized. Such absolute neutralization is the primary object of my invention, and will now be explained in connection with Fig. 2.

I have established the fact that, not only is it necessary to compensate for the feed-back across the grid-plate path of a tube, but the capacity of the tube elements to ground must also be taken into consideration. To stand up under the extremely high voltages existing between the grid and plate of a power amplifier, the insulation of any neutralizing device must be exceptionally good. Such requirements may be met by an evacuated container having therein a plurality of space-separated electrodes and so designed as to exactly duplicate the capacity characteristics of the amplifier tube with which it is to be used.

In Fig. 2, I have illustrated diagrammatically, the various tube capacities that must be compensated. A thermionic'device 80, having a filament 81, a grid 82, and aplate 83, has a certain definite capacity between the grid and plate, represented by a condenser 84.

The grid-to-filament capacity is represented by a condenser 85, the grid-to-ground capacity is represented by another condenser 86, while the filament-to-ground'capacity is represented by a condenser 87. The plate also, has a definite capacity with reference to thefilament, which is its representedby acondenser 8'8, andthe plate-to ground'capacity-is represented by another con'- denser 89. All of these capacities must be dup'licated in theneutr'alizing device, and, for that reason, I have shown such devices as equivalent to thermionic tubes'in the system illustrated in Fig. 1. i

In practicing my invention, I have actually used thermionic tubes as neutralizing devices, and haveobtained very excellent results therewith. Y

In the operation of a radio transmitting system, such as is shown in Fig. 1, the output circuit of the master-oscillator tube is tuned to the frequency of the piezo-electric crystal. Succeeding output circuits may be tuned to this same frequency, or to successively higher harmonics thereof, as explained in myco-pending application, Serial No. 78,115.j The harmonic method is perhaps the better of the two, since the feedback between stages is minimized, and the amplifier is not so prone to oscillate spontaneously. By repeated amplification, a circulatory current of very large amplitude is made to flow in the last output, or tank circuit, as it is sometimes known in the art. By coupling this tank circuit to the radiating device through either a condenser or a properly designed choke coil, a substantially constant frequency may be radiated.

All tendency toward oscillations in the power amplifier circuits is neutralized by the devices 60, since the capacities through which feed-back takes place are exactly compensated. The system will, therefore, radiate no power when the amplifier tubes are blocked, even though the master-oscillator tube continues oscillating.

In certain cases it may be desirable to control the amplitude of oscillations generated by the master oscillator, without altering either filament or plate potential, or changing the grid bias.

Inasmuch as this tube oscillates solely by virtue of the capacity coupling through the grid-plate path, if the effectiveness of this coupling is changed the amplitude of the oscillations will be altered.

In Fig. 3, I have illustrated diagrammatically a. preferred circuit for the control of oscillations in the master-oscillator tube, corresponding to the tube in Fig. 1. A thermionic tube 90, having a filament 91, a grid 92 and a plate 93, is provided with an input circuit comprising a piezoelectric crystal 94 and a radio-frequency choke coil 95. The filament 91 is supplied with power from a source 96, while a source 97 supplied the proper grid bias potential, and plate power'is supplied from another source 98.

An oscillatory circuit, comprising an inductor 99 shunted by a condenser 100, is arranged to be connectedto the plate 93 by a conductor 102,

and to the positive pole of the source 98 by another conductor 103. Connected between a point 104 on the inductor 99 and the grid 92, is a small variable condenser 105. This type of circuit tends to oscillate at a frequency determined by the crystal, the feed-back necessary to sustain oscillations taking place through the grid-plate capacity of the tube. By so choosing the point 104 on the inductor 99 to which is connected the neutralizing condenser that it will always be at a potential equal to, but opposite in phase to the point on the inductor 99 to which the plate is connected, it is possible to impress on the grid of the tube an amount of energy sufiicient to completely stop oscillations. By varying the location on the inductor of the connection from the neutralizing condenser, and, by varying thesize of the condenser, I am enabled toaccurately control the amplitude of oscillations generated.

By my invention, I am enabled to construct a master-oscillator, power-amplifier, radio-transmitting system that has an output at a substantially constant frequency. iBy reason of the perfect neutralization, I am enabled to obtain, with my three-electrode vacuum devices, a transmitting system equipped-therewith which will not radiate an appreciable amount of energy during spacing periods. No detrimental harmonics are present in the radiated wave, thus minimizing, to a considerable degree, interference with other stations.

Although I have illustrated and described herein certain specific'embodiments of my invention, I am fully aware that many other modifications will suggest themselves to those skilled in the art. My invention is accordingly not to be limited except insofar as is necessitated by the prior art and as indicated by the appended claims.

Iclaim as my invention:

1. In an oscillation generator, an electron discharge device having an input circuit comprising a piezo-electric crystal, a tunable output circuit, and reactive means independent of the interelectrode capacity of said tube for incompletely neutralizing the transfer of energy between said circuits.

2. A piezo electric crystal control system comprising an electron tube having grid, filament and plate electrodes, an input circuit interconnecting said grid and filament electrodes, an output circuit interconnecting said plate and filament electrodes, an oscillatory circuit connected with said output circuit, a piezo electric crystal element connected in said input circuit and a variable condenser coupling said input and output circuits for maintaining the current through said piezo electric crystal element below a predetermined value for relieving the strain upon said piezo electric crystal element while sustaining oscillations at maximum amplitude and at the frequency of said piezo electric crystal element in said oscillatory circuit.

3. In a piezo electric crystal control circuit, an electron tube having grid, filament and plate electrodes, an input circuit interconnecting said grid and filament electrodes, an output circuit interconnecting said plate and filament electrodes, a. piezo electric crystal device ground to a predetermined fre'quency characteristic and connected in said input circuit, an oscillatory circuit connected with said output circuit, and means connected between the output and input circuits of said tube for maintaining the current through said piezo electric crystal device below a predetermined value for protecting said device against excessive currents while oscillations of the frequency of said piezo electric crystal device are sustained in said oscillatory circuit.

4. The combination with an oscillator circuit comprising an electric space discharge amplifying device having an output circuit, an input circuit, means comprising the inter-electrode capacity of said device for coupling said circuits, and a crystal device in one of said circuits for controlling th operation thereof, of means for neutralizing said coupling at frequencies including that of the crystal device to a substantial degree but less than sufficiently to prevent oscillation of said circuit.

5. A system according to claim 4, said output circuit presenting tosa'id discharge device the optimum impedance for maximum power output from said device.

6. A system comprising an electric space discharge oscillator having input and output circuits with inherent capacitive coupling between said circuits, a crystal device included in one of said circuits for controlling operation of said oscillator, and means for causing partial neutral ization of said capacitive coupling at frequencies including that of said crystal, to reduce the current flow through said crystal device atsaid frequencies While still allowing the system to oscillate. M

'7. An oscillation generator comprising an electron discharge device having an input and output circuit, a mechanical vibratile frequency conv trolling element in one of said circuits and adjustable means for feeding back from said output circuit to said input circuit of said tube,

energy in substantially phase opposition to the energy fed back through the inter-electrode capacity of the tube.

8., An oscillation generator comprising an electron discharge v device having an input andoutput circuit, a mechanical vibratile frequency controlling element in one of said circuits and adjustable means including a-variable condenser for feeding back from said outputcircuitto said input circuit of said tube, energy in substantially phase opposition to the energy fed back through the inter-electrode capacity of I the tube.

9.- In anoscillationgenerator, an electron discharge device having an input circuit comprising a piezo-electriccrystal, a tunable output circuit, and reactive means independent of the interelectrode capacity of said tube for incompletely neutralizing the transfer of energy between said circuits by way of said inter-electrode capacity.

' DONALD G. LITTLE. 

